REFERENCE laptop repair trobleshooting skills

[jowell]

Let's improve our laptop repair skills by analyzing the circuit

Question: Given the circuit below:

1. if SUSP has 5v before and after power button is pressed, what could be the problem?

2. if SUSP has 0v before and after the power button is pressed, what could be the problem?
3. What is the normal operation of this circuit in terms of voltage readings at SUSP# and SUSP?

UPDATE:
this circuit is commonly used in many laptop designs so understanding this basic circuit will help us solve problems related to discharge circuit and control signals such as syson, susp#, s5_on, etc.

SOLUTION:
1. Q29 is faulty. The drain to source pin of the MOSFET is open, therefore, SUSP = +5VALW
2. There are two potential fault in this scenario: Q29 is faulty. The drain to source pin is shorted or R552 is open.
3. This circuit design is an analog equivalent of a digital inverter with voltage level shifting capability. Since this is an inverter circuit, if 3.3v is present at SUSP#, then SUSP should be 0v. If SUSP# = 0v, then SUSP = +5VALW.

 

[jowell]

troubleshooting an Acer V5-473 w/ board code DAZRQMB18F0 and the voltage measurements as shown in the picture.

This circuit design is found in many laptops and knowing how to troubleshoot them would enhance your capability to fix many laptop problems such as no-power, auto-off and no display problem.

Voltage measurements were done after pressing the power button. Can you analyze the circuit answer the following questions?
(1) Based on the above voltage measurements, which component(s) do you think is the potential faulty component? Why?
(2) Let's assume that all voltage rails are normal but PR118 is open, what voltage can we measure at +5V_S5? Why?
(3) Let's assume that all voltage rails are normal but the drain-source pin of PQ22 is shorted. What voltage can you measure at +5V_S5? What will happen?

SOLUTION

1. PR115, PQ53, PQ28, +15V Rail.
+15V rail must be checked first if it has at least 11v.
PR115 may have changed value.
PQ53 maybe shorted between gate-drain pins or leaky drain-source pins.
PQ28 drain-source pins maybe leaky.
2. +5V_S5 = 5V. Because PR118 is open, then no current will flow from PR115 to ground via PQ28. A voltage of around 9v or higher will be present which will switch on PQ53, therefore measuring 5v at +5V_S5.
3. If drain-source pin of PQ22 is shorted, then +5V_S5 will be shorted to ground. The laptop won't power at all or it may turn off immediately when power button is pressed momentarily. Why, because of PR85, +5V_S5 will have a resistance to ground that is equivalent to 22 ohms. This low resistance will definitely activate the main regulator IC over-current protection.